What are the Data Types in JavaScript?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Split into two groups

   Separate values into primitives and the single non-primitive type, object.

2. Step 2

   List the primitives

   string, number, bigint, boolean, undefined, null, and symbol — seven immutable value types.

3. Step 3

   Understand objects

   Arrays, functions, dates and plain objects are all of type object, stored by reference.

4. Step 4

   Use typeof

   Apply typeof to inspect a value's type, remembering typeof null is the quirky 'object'.

5. Step 5

   Value vs reference

   Primitives copy by value; objects copy the reference, so shared objects reflect edits everywhere.

Code Example

const name = 'Ada';          // string
const age = 36;              // number
const big = 9007199254740993n; // bigint
const active = true;         // boolean
let nothing;                 // undefined
const empty = null;          // null
const id = Symbol('id');     // symbol

const user = { name, age };  // object
const nums = [1, 2, 3];      // object (array)

console.log(typeof name);    // 'string'
console.log(typeof big);     // 'bigint'
console.log(typeof empty);   // 'object' (historical quirk)
console.log(typeof nums);    // 'object'
console.log(Array.isArray(nums)); // true

Follow-up Questions

• Why does typeof null return 'object'?
• What is the difference between undefined and null?
• How are primitives and objects copied differently?
• When would you use a symbol or a bigint?
• How can you reliably check if a value is an array?